Conventional multi-axle assemblies for trucks or trailers may comprise two or more axles. In the case of a two axle arrangement, four leaf springs are typically used. Two leaf springs are on each side of the frame. The remote ends of each pair of springs are usually supported within hanger brackets which are secured to the frame. The adjacent ends of each pair of springs can be supported by a pivotally mounted equalizer. The equalizers are in turn supported by hanger brackets which are secured to the frame.
When the front wheels in the tandem suspension, for example, encounter a condition in the road producing a vertical movement of the front axle, the resultant deflection in the front springs is transmitted in part by the equalizer to the rear springs. This results in an equalizing effect in which any vertical displacement of either axle is distributed between the springs which minimizes the weight differential between the axles.
In the instance of a vehicle equipped with a prior art equalization tandem one problem concerns inadequate and abrupt load equalization from one axle to another. As one end of the equalizer rotates upwardly at the point at which the spring end contacts the leaf spring, the other end abruptly rotates downward. This action causes abrupt and unequal moments about the center pivot point of the equalizer; inequality increases as the range of equalizer movement increases.
The end results of improper load equalization are numerous. If the axle which encounters bumps and overloads is unpowered, a loss of traction can occur on the powered axle. The suspension components, and springs in particular, are subjected to higher stresses and therefore their service life is shortened appreciably. As the leaf springs are subjected to overload they progressively lose capacity to absorb energy and therefore transmit more energy through the mounting brackets to the vehicle frame. Increased input of energy into the vehicle frame can often contribute at given loads, speeds and highway conditions to damage to the vehicle. Further, wherever an unequal load distribution occurs within a tandem suspension, the axle which is overloaded transmits its load to the roadway in a manner that can be detrimental to the roadway. The foregoing arguments also apply to suspensions comprising more than two axles.
Torsion axles have been known for many years. Torsion axles have proven to be extremely popular primarily because if one wheel hits a bump or rut, it can react independently of the other wheel, which may not have hit a bump or rut at the exact same time. This torsion axle concept therefore operates to keep a trailer or the like moving as straight as possible behind a towing vehicle and absorbs some of the shock of the road over which it is passing with an independent suspension. This can be contrasted with a straight axle situation where if one wheel were to drop into a rut or the like and be slowed down for that reason while the wheel on the other side of the trailer did not have the same experience at the same time, the trailer would tend to turn somewhat to allow the wheel that is on the flat part of the road to move forward while the wheel that is in the rut would be restrained, therefore causing the axle to be not perpendicular with the direction of towing of the vehicle itself, assuming in this example that the trailer was being towed on a straight portion of the road.
The two aforementioned torsion axles are typically constructed of a square axle in cross section with elongated rubber members all disposed within a larger tube. One of the most common and popular torsion axles is a TorFlex® rubber torsion suspension system by Dexter Axle Company. This mentioned torsion axle has independent and separate stub axles on each side to enhance the independent aspect of such an axle.
Representative of the art is U.S. Pat. No. 6,340,165 which discloses a vehicle suspension system assembly for attachment to a vehicle frame having a torsion axle and at least two ground engaging wheels operatively rotatably attached to each respective end of the torsion axle has an attachment member which is adapted to be attached to the frame. An arm is operably pivotally attached to the attachment member along an axis. A torsion axle is received in a torsion axle receiving portion of the assembly at a first distance spaced from the aforementioned axis for selectively receiving the torsion axle. An air bag is operatively disposed between the frame and the arm, the air bag being spaced at a second distance from the axis. A linkage element of variable length, which can be a shock absorber, is operably attached to the frame and to the arm whereby the arm will be held at a predetermined position with respect to the frame and yet allow the arm to pivot about the axis. In a preferred embodiment, the air bag is spaced further away from the pivotal axis of the arm than is the torsion axle.
Further representative of the art is U.S. Pat. No. 7,753,400 which discloses a multi-axle leaf spring suspension having a compliant equalizer. The leaf springs are mounted one behind the other on each side of the vehicle. Adjacent ends of the leaf springs are pivotally attached to the compliant equalizer. The compliant equalizer is pivotally mounted to a vehicle frame. The compliant equalizer comprises a pair of arms that are pivotally connected to each other in a scissor-like fashion. One end of each arm is pivotally connected to the end of a leaf spring. The other end of each arm is cooperatively arranged to contain and compress a spring member between them. The compliant equalizer reduces the magnitude of shocks that would be otherwise transmitted to the suspension and frame by absorbing them through compression of the spring member.
What is needed is a multi-axle vehicle suspension system having at least two torsion axles, each torsion axle coupled to a vehicle frame, and further having a pivotal member pivotally coupled to the vehicle frame and pivotally coupled to each torsion axle. The present invention meets this need.